GreasePencil goes trough a mesh in wireframe mode 2.79
Okay, the Related sidebar just provided one of the workarounds once the question was posted:

See background image through solid shading in edit mode

it is possible to remain in the Solid mode and then simply switch the background image from being on the back to be on the front. Then some opacity tweaks will do the rest.

Still if someone could point out where was my mistake in the default set up it'd be much appreciated as it will help me understand the GP workflow a bit better.